Bitters, Digestifs, and Apéritifs (BD&A's) are similar to Liqueurs, but much bitterer in taste and usually without the extra sweetness to them. Like Liqueurs, BD&A's were originally created as medicines made from herbs, roots, twigs, vegetables, fruits, and berries. To lessen the harsh taste they were sweetened, just not as much as their cousins, liqueurs. There is not always a hard and fine line between Bitters, Digestifs, Apéritifs, and Liqueurs, and some can be classified in one, two, three, or all four categories.
Bitters come in several styles and many were formerly sold as patent medicines. There are the ones like Angostura Bitters from Trinidad, and New Orleans's Regan's Orange Bitters and Peychaud's Bitters which are used in small amounts, added to a cocktail to bring together the flavors and cut the sweetness. Then you have ones that are sold in tiny, 2/3 oz bottles such as Underberg from Germany, which you knock back in a quick gulp after a meal to settle your quaking belly. Many others I also classify under apéritifs and digestifs when you cut them with seltzer and ice.
Apéritifs tend to be drunk more before a meal to stimulate the appetite. The term is from the Latin word aperire, which means to open, the digestive tract. They tend to be relatively low alcohol compared to other spirits and are in the 16% to 24% abv range. They tend to be made predominantly from wine derivatives combined with medicinal herbs and roots. Well known ones are Vermouth, a French or Italian fortified wine, Italian Campari, Pernod from France, Amer Picon from France, and the unique tasting Cynar made from artichokes. They can be drunk chilled straight, or mixed with seltzer or other beverages to make them more approachable in taste.
Digestifs are used to comfort the stomach after a large meal and to aid digestion and tend to be made with a higher sugar and alcohol level than Aperitifs. Examples are herb based liqueurs like Chartreuse, Galliano, Anisette and Sambuca, Drambuie, Benedictine, Tuaca, Fernet Branca, and the infamous, in-between German concoction Jagermeister which starts off sweet but ends up more like a bitter. Some are meant to be sipped in from small glasses, others in larger amounts cut with seltzer, they can be chilled or served on the rocks, and even added to coffee.
BD&A's can be made in the same ways as liqueurs. Through maceration which is when you take the botanical ingredients and soak them in pure grain neutral spirits. Then after several weeks or months as sufficient to obtain the maximum flavor, you remove the solids and filter until clear. This method works well with delicate fruits whose flavor and aroma could be destroyed or harshened through other methods. In this category you also have infusion which is basically like steeping a tea of dried botanicals in spirits to get the flavors out and then filter.
Percolation is when you drip heated spirits through the ingredients over and over until you have removed the desired flavors, and then filter clear. This is used a lot with botanicals like coffee, cocoa nibs, and vanilla beans.
Distillation is when you use a pot still and take the dried botanicals and add them to spirits to soak for awhile. Then you distill the whole mess, leaving you with clean spirits which have picked up the aromas and the essence of the flavor of the botanicals. This is similar to how gin is made.
After going through one or more of these processes the results are mixed according to secret recipes, cut to a milder alcoholic strength, and sweetened. Then they are aged to bring together and smooth out the taste and aromas. Aging may be in stainless steel or glass so that you get no additional flavors, or in wooden casks and charred casks to pick up additional tastes and aromas.